The fees for sending are rather low if any (i.e. miner fees from GDAX/Coinbase only), it's the buying fees there that aren't. If you want to stay on Coinbase I suggest wiring money (DO NOT PAY 4% credit card buy pls!) and then sending it over to GDAX as it's the same company. Then even if you market buy you'll pay only 0.25%.

Stay safe and stay in profits